Method of mounting screens for cathode-ray tubes



Oct. 13, 1953 H. B. LAW 2,654,940

METHOD OF MOUNTING SCREENS FOR CATHODE-RAY TUBES Filed June 10, 1950 INVENTOR Patented Oct. 13, 1953 UNITED STATES PATENT OFFICE METHOD OF MOUNTING SCREENS FOR CATH'ODE-RAY TUBES ware Application June '10, 1950, Serial No. 167,410

7 Claims.

This invention relates to screens in cathode ray tubes. Screens of very fine mesh are used as electrodes to produce a potential field adjacent the target under scansion by the beam, the electrons of which pass through the interstices of the screen to approach or land on the target.

This application is a continuation-in-part of my copendi-ng application Serial No. 585,925, filed March 31, 1945, now abandoned, and assigned to the same assignee as the present application.

Screens of the type described have been variously made of fine wire, such as by weaving wires into a sheet metal cloth as fine as 400 per square inch. The metal cloth itself is incapable of maintaining a planar or other definite surface because of its thinness and flexibility. It has therefore been the practice to stretch the metal cloth as taut as possible over themetal frame and secure it thereto, the cloth, due to its woven .character, yielding sufficiently to permit this.

To obtain screens of still finer mesh and also to improve upon the method of construction, I have devised an electrolytic process for making :copper target screens of only a few tenthousandths of an inch thickness with up to .1000 meshs per inch, as set forth in my application, filed ,April 1%, 1944, Serial No. 531,008. These electrolytic screens have proven very satisfactory in use astargets. However, the problem of mounting these exceedingly thin and fine mesh screens on frames has proven of difficult solution. Old processes of mounting have left much to be desired.

It is an object of this invention to provide new screen units or assemblies and novel methods of mounting the screens in the frames.

Other objects will appear in the following specification, reference being had to the drawings, in which:

Fig. 1 is a sectional elevation of a screen unit assembled for heat treatment in the preferred method of securing a taut screen.

Fig. 2 illustrates the final condition of the parts after using another method of tightening the screen.

Fig. 3 is a sectional elevation illustrating another embodiment of my invention.

In the drawing, the figures are broken away, the parts are enlarged and the perforations or openings of the screen are omitted to permit adequate illustration.

Referring to Fig. 1, an extremely thin electrolytic or equivalent screen I, a few ten-thousandths of an inch in thickness, is placed on a metal ring 2 of equal or somewhat greater dimension and a ring 3 is placed on the screen, as shown in Fig. 1. The two rings, with the sandwiched screen, may then be fastened together by spot or line welding or otherwise, with the screen I in as taut condition as practical, though due to the extreme thinness of the screen it will still be somewhat wrinkled and nonplanar, as already indicated. This assembled unit is then placed in a suitable oven and is heated between 800 to 1000 C. in a suitable vacuum or inert atmosphere up to ten minutes. The heating shrinks and tightens the screen while in the heated condition. At this temperature to which screen I is heated, the screen can be observed to tighten across the mounting rings 2 and 3. It has been found that this shrinking or tightening is due to the fact that the copper metal tends to soften; that surface forces are set up, and the surface tension of the material causes the metal of the wire screen to draw together resulting in a shrinking or tightening of the wire mesh across the mounting rings 2 and 3. This phenomenon resulting from the action of surface forces has been observed not only in. fine mesh screens but also in metal diaphragms. With electrolytic copper, which is used in the mesh I, described above, there is still a further characteristic of the metal, which tends to draw the screen I more tightly across its supporting ring. This is, that electrolytic copper tends to change to a greater density upon heating, which produces an additional tightening of the copper mesh I across the supporting rings 2 and 3.

Although the density change is usually completed after one heating, the above process of tightening a fine metal mesh screen across its supporting rings can be repeated any number of times that it becomes necessary to tighten the mesh because the surface tension forces continue to act. If, because of use, wire mesh I becomes dented or ceases to be sufficiently taut across the supporting rings 2 and 3, the tightening process described may be repeated to restore the metal mesh I to its original tautness.

This tightening or shrinking of the wire mesh is not an instantaneous action, but is one that occurs over a period of several minutes. When the ring'assernbly, shown in Fig. 1, is first placed into the heated oven, the mesh screen I will, at first, tend to sag more across the center of the rings 2 and 3, due to the fact that the coefficient of expansion of the copper of mesh I is greater than that of the nickel rings 2 and 3. However, as the heat treatment is continued above 800 C., the surface tension forces, set up by the heating of the copper mesh, will cause the wire mesh I to draw up and tighten across the mounting rings 2 and 3. This shrinking or tightening of the copper mesh by the heat treatment occurs within minutes, if the temperature is held between 800 to 1000. During the heating process, when it is noted that mesh I has tightened across the mounting frame rings 2 and 3, the heating can be discontinued. The assembly is cooled slowly in vacuum where the greater coeflicient of expansion of the copper metal mesh I will cause an additional tightening of the wire mesh across the nickel support rings 2 and 3, as the assembly is cooled to ambient temperature. The tightening screen is then removed from vacuum.

To prevent oxidation of the copper mesh I during the firing, it is preferable that the heating of the mesh assembly is performed in a suitable vacuum or inert atmosphere. In the making of a screen assembly for a successfully operated television pickup tube, a fine mesh copper screen of thickness between 0.7 and 0.9 mils is used. This screen and its support ring-assembly is heated in a metal container in vacuum by a high frequency induction field to a temperature of 1000 C. for approximately 8 minutes.

The thickness of solid diaphragms and the cross-sectional area of individual wires of the metal mesh I is a determining factor in the use of this method of tightening. For example, in fine mesh screens the thickness effect results from pressure being applied by surface tension on a cross-section of wire, the area of which increases with the square of the radius while the surface, and therefore the surface tension forces, increase as the first power of the radius. As a result very fine wires tighten rapidly, but as the wire size increases the tightening effects occur very much slower. It has been found that wires on diaphragms of more than 1 mil thickness begin to tighten with difficulty.

The tightening of a fine mesh screen or diaphragm across its supporting rings is also a function of the temperatures used. At high temperature, the forces of surface tension act more quickly so that less time is required in firing the mesh. At lower temperatures, the firing time becomes longer as the shrinking or drawing up of the mesh is relatively slow. With a copper screen or diaphragm, firing temperatures below 800 C. are too slow and impractical. It is obvious, however, that the temperatures of the firing should be kept below the melting temperature of the metal used.

This characteristic of certain metals to draw up or shrink due to surface tension action has been observed with other metals as well as copper. For example, it is known that thin gold sheets exhibit a surface tension at some 100 C. below the melting point of gold. I have also observed this characteristic with silver, nickel and aluminum sheets.

The tightening of a fine silver mesh or diaphragm across the supporting ring is a rather critical process. When a silver mesh or film is fired in a vacuum it is difiicult to prevent the silver metal from melting and also evaporating before the forces of surface tension have acted to draw the mesh or diaphragm tightly across its supporting ring. However, if the tempera ture is carefully controlled, silver may be successfully used for a mesh or diaphragm and tightened by the process described.

It has been found that rings 2 and 3 braze lightly to the screen I during the tightening process provided the material of rings 2 and 3 does not oxidize during the heating process. Rings 2 and 3 may be made of nickel, steel, or if plated lightly, with copper or silver, may be made of an alloy containing chromium.

It is, of course, preferable that the coefficient of expansion, of rings 2 and 3 be less than that of the wire mesh screen I. If the coefficient of expansion of rings 2 and 3 were of greater than that of the mesh screen I, it is obvious that the cooling of the mesh assembly after the step of firing, will produce a sagging of the screen I between the supporting rings 2 and 3.

This method of tightening metal screens or diaphragms will operate with all metals which soften to an extent where the surface tension of the metal draws the portions of the mesh or diaphragm together to produce a tight planar surface across the supporting rings. This method of tightening fine mesh screens or diaphragms may also be used with screens formed by the spraying or the evaporation of metal, or screens etched from solid sheets of metal.

Another, though not so satisfactory, method of tightening the screen into planar position after assembling the parts of Fig. 1 and welding them together is to bend the ring 2 inwards toward the center of ring 3, as indicated in Fig. 2. This may be done by hand or otherwise. It will be apparent that the inturned bent portion 4 of ring 2 will stretch the screen I taut and planar if the outer portion of ring 2 is held in planar position, as by the stiff ring 3 or otherwise.

Another method of making a taut screen is illustrated in Fig. 3. In that embodiment the screen I is placed over a ring 5 resting on a base 6 and another ring I is placed therearound, with the peripheral portion of the screen sandwiched between the two rings so as to obtain a preliminary tightness of the screen. In accordance with this method, the final stretching is obtained by thermal expansion of the base 6 and certain of the parts must have different coefficients of expansion. For example, the rings 5 and I may have the same coeflicient of expansion and the screen I should have a greater coefficient of expansion than the rings. The base 6 then should have a still greater coefficient of expansion. Various metals may be used to obtain this difference in expansion and contraction, but by way of example, I will say that I have secured very good results by making the rings 5 and I of metal consisting of 50% nickel and 50% iron. The screen I may be made of copper and the base 6 of aluminum or an aluminum alloy. After the parts have been assembled as in Fig. 3, they are placed in an oven and the assembly is then heated to around 530 C. in a vacuum or inert atmosphere. The heating causes the base 6 to expand and stretch the less expansive rings. The stretching of these two rings is great enough to exceed their elastic limit and they take a permanent set from this expansion. Then, when the parts are cooled down, the screen I remains in taut planar condition due to its greater coefiicient of expansion and to the permanent change in diameter of the rings between which it is sandwiched. After the parts are cooled down, the base 6 is removed.

Various modifications may be made without departing from the spirit of the invention.

I claim:

1. The method of making a screen unit for cathode beam tubes, comprising sandwiching the peripheral edge of a thin copper screen less than one thousandth of an inch in thickness between two nickel rings, welding said rings and screen periphery together, and heating said screen and rings at a temperature of 800 C. to 1000 C. for a time up to minutes until said copper screen tightens, and cooling said screen and ring to ambient temperature.

2. The method of making a screen assembly formed from a metal mounting ring and a thin Wire screen less than one thousandth of an inch in thickness having a larger coefiicient of expansion than that of the mounting ring, said method comprising the steps of, fastening the peripheral edge of said screen to said mounting ring, heating said ring and screen to a high temperature below the melting point of the screen metal until the screen tightens across the mounting ring, cooling said ring and screen so that the greater expansion coeilicient of said screen results in a further tightening of said screen across said mounting ring.

3. The method of making a screen assembly formed from a nickel mounting ring and a thin copper wire screen of less than one thousandth of an inch in thickness, said method comprising the steps of, fastening the peripheral edge of said screen to said mounting ring, heating said ring and screen above 800 C. and below the melting point of copper until the screen tightens across the mounting ring, cooling said ring and screen to ambient temperature so that the greater expansion characteristic of the copper screen results in a further tightening of the screen across the nickel ring,

4. The method of making a screen unit for cathode beam tubes in which the screen assembly is formed from a metal mounting ring and a thin copper wire screen of less than one thousandth of an inch in thickness, the copper wire screen having a larger coefiicient of expansion than that of the mounting ring, said method comprising the steps of, fastening the peripheral edge of the tube screen to said mounting ring, heating said ring and copper screen above 800 C. and below the melting point of the copper screen until the copper screen tightens across the mounting ring,

cooling said ring and screen to ambient temperature so that the greater expansion characteristi of the tube screen results in a further tightening of the screen across the ring.

5. The method of making a screen unit for cathode beam tubes, comprising fastening the peripheral edge of a thin metal screen less than one thousandth of an inch in thickness to a metal ring having a coefiicient of expansion less than that of the metal screen, heating said screen and ring to a high temperature less than the melting point of the screen and until the screen in the heated condition tightens across the ring, and cooling said screen unit to ambient temperature.

6. The method of making a screen unit for cathode beam tubes, comprising fastening the peripheral edge of a thin copper screen less than one thousandth of an inch in thickness to a nickel ring, heating said screen and ring to a high temperature somewhat less than the melting point of the copper screen and until the screen in the heated condition tightens in the ring, and cooling said screen unit to ambient temperature.

7. The method of making a screen unit for cathode beam tubes, comprising sandwiching the peripheral edge of a thin metal screen less than one thousandth of an inch in thickness between two metal rings having a coefficient of expansion not greater than that of said metal screen, welding said rings and screen periphery together, heating said screen and rings to a high temperature less than the melting point of said screen, continuing the heating at this temperature for a time sufficient for said screen to tighten in said rings, and cooling said screen and rings to ambient temperature.

HAROLD B. LAW.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Arnoldy Oct. 14, 1947 OTHER REFERENCES Number 

